1. Field of the Invention
The present invention relates generally to bioreactor systems and more particularly to methods for their use in the chemoautotrophic synthesis of chemical products from carbon captured from industrial waste streams.
2. Description of the Prior Art
The reliance on petrochemicals as a primary feedstock for the creation of products such as plastics, cosmetics, lubricants, adhesives, paints, transportation fuels and many other commodity, specialty, and fine chemicals is increasingly under scrutiny due both to the resultant production of greenhouse gases, and the high and variable cost of the petroleum feedstock. Renewable chemicals derived from microbial sources offer environmentally sustainable alternatives to those derived from fossil resources.
Microbes require a source of carbon in order to live, grow and produce chemical products. Many industrial sources produce large amounts of carbon, primarily in waste gases as one or both of the carbon oxides, carbon dioxide (CO2) and carbon monoxide (CO). Carbon oxides from industrial sources are primarily produced by the combustion of fossil fuels and/or chemicals and are classified as greenhouse gases due to their contribution to deleterious environmental conditions. Thus, capturing carbon oxides from industrial gas effluent is both a potentially cheap and scalable way to obtain carbon for biologically mediated chemical production, and a way to reduce the amount of carbon dioxide released into the atmosphere.
Cement manufacture is a major source of atmospheric carbon dioxide, as well as other greenhouse gases. In cement production, a mineral feedstock is progressively heated to increasingly higher temperatures, causing a succession of chemical reactions to take place. One of these reactions is calcination, also referred to as calcining, in which the carbonate-bearing minerals within the feedstock decompose, releasing carbon dioxide. The decomposition of calcium carbonate, for example, is represented as follows:CaCO3→CaO+CO2.Further reactions at even higher temperatures yields a “clinker” which is a sintered mass that is then ground to an appropriate fineness for cement.
In addition to the carbon dioxide generated by the carbonate mineral decomposition in calcination process, above, cement production also generates carbon dioxide in further ways. For instance, mechanical processing such as crushing and grinding and the high temperatures used to produce the final clinker, all tend to be achieved by the combustion of fossil fuels.
Accordingly, cement manufacturing produces at least two carbon waste streams. The first stream principally comprises the carbon dioxide from carbonate decomposition. Water vapor (steam) is a major impurity in this first stream as water in the raw mineral feedstock is also driven off. Other impurity gases vary with the particular composition, purity and contaminants in the raw mineral feedstock.
The second carbon waste stream comprises carbon dioxide, carbon monoxide, and other gases produced by the combustion of feedstock fuels for heating and running motors for conveyors and grinders, for example. These fuels are often fossil fuels, but can also be fuels derived from the combustion or gasification of biomass, waste materials, or other fuel sources. The exact composition of this gas in the second stream is dependent upon the composition of the fuel feedstock in use.
The compositions of both the first and second gas streams are also affected by the uses of air and sometimes the use of other gases. For example, both streams tend to include nitrogen and oxygen from the atmosphere since calcining is typically performed in air and air typically provides the oxygen for combustion. Cement plant flue gas often also contains oxides of sulfur and nitrogen, referred to generally as SOx and NOx, as well as hydrogen sulfide and other greenhouse gases.
The production of renewable chemicals involves the capture of carbon oxides and their incorporation into chemical products. Microbial systems offer environmentally sustainable, greenhouse-gas-sparing, and highly energy, water, and carbon-efficient, chemical manufacturing processes for the capture of carbon oxides. Microbial chemical production can be local in most areas, and can be co-located with carbon releasing industries such as cement manufacturing. By using carbon captured from the waste streams of industry (whether gas, liquid or solid waste), it is possible to produce truly carbon neutral, renewable, replacements for petroleum products. This also reduces dependence on imported fossil energy resources.
Fermentation is a well-known process wherein chemical compounds such as sugars are provided as feedstock. In fermentation, sugars are broken down to produce commercially useful, but lower energy, products like ethanol. Such chemical feedstock provides the source of carbon, essential for building up new compounds and allowing microbial metabolism and growth (anabolism), and the chemical bond energy that is used to drive the process energetically. This type of metabolism is called heterotrophic and has historically dominated the use of diverse bacteria and fungi to make chemicals useful to society.
Another type of microbial metabolism, called autotrophic, refers to the use of inorganic carbon sources, primarily captured carbon dioxide, as the primary source of carbon. These inorganic carbon sources provide the essential carbon source but embody significantly less chemical energy than sugars used in heterotrophic growth. Photoautotrophic organisms, including the green plants and algae, use light energy to drive the capture of carbon dioxide. The term “chemoautotrophic” pertains to organisms that derive both their energy and their carbon from inorganic chemical sources.
Chemoautotrophic metabolism describes a metabolic mode in which the organism uptakes inorganic carbon, such as by capturing carbon dioxide, as a primary carbon source, and obtains energy from a chemical source, such as by oxidizing hydrogen. Chemoautotrophic metabolism is primarily found in a number of bacteria, including, but not limited to purple non-sulfur (PNS) bacteria such as Rhodobacter capsulatus, Rhodobacter sphaeroides, Rhodpsuedamonas palustris, the betaproteobacteria, such as Ralstonia metallidurans (Ralstonia eutropha.), the pseudomonas, such as Pseudomonas carboxydovorans, the methanogenous bacteria, such as Methanobacterium thermoautotrophicum, the betaproteobacteria, such as Ralstonia metallidurans, Ralstonia eutropha, the acetogenous bacteria, such as Acetobacterium woodii, or other microbes which express both an uptake hydrogenase and a carbon dioxide fixation metabolism, whether endogenous or introduced through genetic manipulation, mutation, selection or directed evolution, such as Escherichia coli, Anabaena, Bacillus subtilus, etc. In many cases these microbes are capable of heterotrophic as well as phototrophic metabolism, or mixed metabolism using both sources of energy and carbon. In some cases molecular hydrogen (H2) is used as the energy source, and carbon dioxide is used as the carbon source. Carbon monoxide can also act as a possible energy source and as a possible carbon source, but carbon monoxide generally works best in a mix with hydrogen and/or carbon dioxide and/or oxygen, due to its toxicity.
Gasification is a process where biomass, fossil fuels, or other carbon containing materials are subjected to high temperature and a restricted supply of air or oxygen in a controlled reactor called a gasifier. This process, referred to as pyrolysis when oxygen is not provided, produces a number of gasses, principally including carbon monoxide and hydrogen but may also produce one or more of carbon dioxide, water vapor, methane, ethylene, and ethane. Pyrolysis at lower temperatures is known as torrefaction. Gas streams produced by gasification have large amounts of carbon monoxide and can be further processed to convert the carbon monoxide and water into carbon dioxide and hydrogen via several processes referred to as reforming processes, notably steam reforming. Reforming techniques include, but are not limited to, steam reforming, catalytic reforming, and biologically mediated reforming such as biocatalysed electrolysis or fermentative hydrogen production. Furthermore, molecular hydrogen and carbon monoxide are major components of syngas, where varying amounts of carbon monoxide and molecular hydrogen are generated by gasification of a carbon-containing fuel. For example, syngas may be produced by cracking the organic biomass of municipal waste, waste water solids, waste woods, timber, plastics, and non-biodegradable carbon containing materials, to generate precursors for the production of fuels and more complex chemicals.